I. Field of the Invention
This invention relates to a digital communication system. In particular, it relates to a transfer mode conversion technique between communication systems having different transfer speeds with respect to each other.
II. Description of the related art
The examples of prior art are described referring to FIGS. 1, 2 and 3. FIG. 1 is a schematic diagram showing a connection of the circuit between a digital communication system (hereinafter referred to as "PHS: personal handy phone system") network 100 using 4 channel speech circuit by 32kbps ADPCM (adaptive differential pulse-code modulation) in a radio zone and a public fixed communication network 200 constituted of ISDN (integrated services digital network).
Numeral 1 depicts a terminal device of the PHS system, and 2 depicts a base station for performing communication with the terminal device 1 by radio channels. The base station 2 is connected to a switching center 3 through a 2B+D circuit which includes two B-channels with a transfer speed of 64 kbps and one D-channel with a transfer speed of 16 kbps that is a basic rate access interface of a user-network interface of ISDN.
Numeral 4 depicts another switching center constituting the public fixed communication network 200 by ISDN together with the switching center 3.
FIG. 2 is an illustrative view showing operation of the base station of the example of prior art.
A speech channel 6 of 32 kbps ADPCM of the terminal device 1 is connected to the 2B+D circuit 5 of the ISDN user-network interface of the switching center 3, and to achieve connection status as shown in FIG. 2, where the control proceeds to receive a speech data transferred in a radio zone with TDMA/TDD by a transmitting/receiving equipment 21, to separate the data at every speech channel basis, and to take out 32 kbps ADPCM data respectively as B'1, B'2, B'3 and B'4.
Thus taken out data are respectively converted into 64 kbps PCM data B1 to B4 through the PCM converters 22 (PCM converter No. 0 to No. 3) which convert 32 kbps ADPCM to 64 kbps PCM. These data are grouped into two set of groups (for example, a set of B1 and B2, and a set of B3 and B4) in order to being transferred on two B-channels of subscriber's line having the 2B+D structure, and are delivered to S interfaces 23 (S interface No. 0 and No. 1). On the other hand, a call from switching center 3 travels through a reverse route from the S interface 23, and is connected to a radio channel through the PCM converters 22 which convert 64 kbps PCM to 32 kbps ADPCM. FIG. 3 shows more detailed configuration of the one of the PCM converters 22. The 32 kbps ADPCM data transferred from the transmitting/receiving equipment 21 is once stored in the T (Transmitting: the data flow from a terminal device to a switching center) side register A 221, the data is converted to 64 kbps PCM by the PCM converter 222 and the converted data is stored in the T side register B 223. The stored data in the T side register B 223 is then output to the S interface 23.
The similar procedures are taken for the data flowing from the S interface 23 to the transmitting/receiving equipment 21 using the R (Receiving: the data flow from a switching center to a terminal device) side register B 224, the PCM converter 225 which converts 64 kbps PCM to 32 kbps ADPCM and the R side register A 226.
The ISDN subscriber's line interface has a transfer capability of 64 kbps in a B channel. In contrast to this, a transfer capability of a channel in the PHS system is only 32 kbps. Therefore, the conventional data transfer technique which requires conversion of transfer speed between 64 kbps and 32 kbps cannot achieve an effective utilization of the B channel.
It is required four sets each of PCM converters corresponding to four speech channels, e.g. four sets for converting 32 kbps ADPCM to 64 kbps PCM and another four sets for converting 64 kbps PCM to 32 kbps ADPCM in the base station, and it causes equipment of the base station to be a larger size.
It is expected in the future technical trend of the PHS system that the radio channel will become a half rate (16 kbps) or a quarter rate (8 kbps), and it means that the number of speech channels, including the subscriber's lines between the base station and the switching center, comes twice as many time the half rate is realized. Therefore, expenses of capital investment for new installations of the subscriber's lines will become a great amount.